The present invention relates to the use of a transistor differential pair and its use as the input to many integrated circuit amplifiers. It has specific application to Radio Frequency Integrated Circuit (RFIC) amplifiers and double balanced (Gilbert cell) mixers, as well as many other possible uses. Such mixers are used in numerous applications, including, but not limited to, upconverters and downconverters for cellular or other wireless communications.
A conventional differential amplifier is shown in FIG. 1, and a typical double balanced mixer is shown in FIG. 2. Though these examples show bipolar transistors, the analysis is identical for Field Effect Transistors (FETs i.e., MOSFETS, MESFETS, JFETS . . . ), and this invention is applicable to both bipolar and field effect transistor circuits. These examples also show resistive loads RL.sub.1, RL.sub.2, RL.sub.3 and RL.sub.4, but there are a wide variety of loads that could be used, for example, current combiners, transformers, inductors, tuned networks and similar circuits.
As seen in FIG. 1, transistors Q.sub.1 and Q.sub.2 have their emitters tied and supplied through current source CS1. An input voltage, VIN.sub.1, is applied across their bases, and a collector voltage VCC.sub.1 applied through load resistors RL.sub.1 and RL.sub.2 determines the output signal VOUT.sub.1 as the amplified differential of VIN.sub.1.
In FIG. 2, the Gilbert cell has transistors Q.sub.5, Q.sub.6, Q.sub.7, and Q.sub.8 arranged with the emitters of Q.sub.5 and Q.sub.6 tied together and the emitters of Q.sub.7 and Q.sub.8 tied together. The bases of Q.sub.6 and Q.sub.7 receive one side of local oscillator signal LO_IN.sub.1, and the bases of Q.sub.5 and Q.sub.8 receive the other pole of LO_IN.sub.1. The collectors of Q.sub.6 and Q.sub.7 are tied as one pole of the output signal VOUT.sub.2, and the collectors of Q.sub.6 and Q.sub.8 are tied as the other output polarity of VOUT.sub.2. The two tied collector pairs are connected respectively through load resistors RL.sub.3 and RL.sub.4 to the collector voltage supply VCC.sub.2.
The tied emitters of Q.sub.5 and Q.sub.6 are connected to the collector of transistor Q.sub.3 and the tied emitters of Q.sub.7 and Q.sub.8 are connected to the collector of transistor Q.sub.4. Input signal VIN.sub.2 is connected across the bases of Q.sub.3 and Q.sub.4. The emitters of transistors Q.sub.3 and Q.sub.4 are tied to a constant current source CS.sub.2.
As can be seen, the transistors Q.sub.3 and Q.sub.4 act as a differential amplifier component of the Gilbert cell circuit, which on the whole acts to multiply the voltage VIN.sub.2 by the local oscillator signal LO_IN.sub.1 to provide the output signal VOUT.sub.2.
The small signal gain of circuit in FIG. 1 is: gm*RL.sub.1. (Small signal is defined to be about 10 mV or less.) For input signals larger than about 50 mV, the differential input pair suffers from linearity problems. The typical solution to this problem is to "degenerate" the input pair. This is done by adding impedance Z.sub.1 as shown in FIG. 3 or FIG. 4, which show the amplifier sections only, for simplicity.
In FIG. 3, two identical impedances Z.sub.1 have been interposed between the transistor emitters and the current source CS.sub.1. In FIG. 4, two current sources CS.sub.3 and CS.sub.4 are provided, one tied to each transistor emitter, with the two emitters coupled through an impedance equal to twice Z.sub.1.
The equivalent gm of the input pair changes to: Gm=1/(1(/gm)+Z.sub.1). If Z.sub.1 is large compared to 1/gm (which is typically the case), than the gain is dominated by this degeneration impedance. The gain is also less than it was without the degeneration, but the input dynamic range will be greater.
The circuit in FIG. 3 tends to have less noise than that of FIG. 4, because the noise contribution from the bias current source CS.sub.1 is common-moded out. But, FIG. 3 has the disadvantage in that, if Z.sub.1 is resistive, DC current flows through this impedance, and a substantial voltage drop will result, reducing the dynamic range. Also, since DC current must flow, Z.sub.1 can not be capacitive. One common solution to this problem is to make Z.sub.1 purely inductive, or use the circuit in FIG. 4. The problem with using the FIG. 3 circuit with Z.sub.1 purely inductive is that at lower frequencies, the impedance becomes less, and therefore gain becomes greater. This causes lower frequency noise, especially at the image frequency, to be gained up more than the signal frequency of interest.
The circuit in FIG. 4 solved the problem of DC current flowing through the degeneration by adding a second current source. The big problem with this approach is that the noise from the current sources is no longer common-moded out, and can add substantial noise to the entire amplifier and/or mixer. Thus, prior degeneration efforts have been plagued with one detrimental side effect or another, leaving an unresolved need.